It is little wonder that the ever-smiling Chinese prime minister, Wen Jiabao, has turned his grin on to full beam since last week's meeting with George Bush.

Not only did the dapper Beijing technocrat nimbly dodge the vexed issue of the bilateral trade deficit, he also came away from Washington with an unexpectedly generous souvenir from the US president: a clear-cut warning that Taiwan should not press for independence.

In the most public rap on the knuckles ever meted out by an American leader to an ally in Taipei, Bush criticised Taiwan's president, Chen Shui-bian, for threatening to destabilise relations across the strait.

Standing beside the Chinese leader, Mr Bush said: "We oppose any unilateral decision by either China or Taiwan to change the status quo. And the comments and actions made by the leader of Taiwan indicate that he may be willing to make decisions unilaterally to change the status quo, which we oppose."

Those two sentences - which amount to a condemnation of Mr Chen's recent call for a referendum and a revision of the constitution - clearly came as a shock to many listeners. Several US commentators expressed disappointment that the leader of the free world could slap down the democratically elected head of Taiwan, while sharing handshakes and warm words with the premier of China's totalitarian government.

But they should not have been so surprised. Mr Bush's statement was entirely consistent with a foreign policy that - despite all of the president's talk of "good" and "evil" - has been shaped by realpolitik rather than principle.

Mr Chen had to be rebuked because he is deliberately provoking Beijing in order to get re-elected during the presidential poll next March. As was the case during the past two elections, he is clearly hoping that an angry backlash from the mainland will polarise opinion in Taiwan and increase support for his pro-independence party.

On the previous two occasions, this was tolerated - even encouraged - by Washington, but this year the US cannot afford another Taiwan crisis. The Pentagon is too embroiled in Iraq and too concerned with North Korea to dispatch an aircraft carrier to the region as it did during the last big war of words in 1996.

The US also has a lot more reasons to want to maintain good relations with China, which is an increasingly important economic and diplomatic player. Trade between the two nations has increased 50-fold in the past 25 years. Beijing is taking a leading role in trying to broker a peace deal between the US and North Korea. It has also thrown its support behind Mr Bush's "war on terror".

So Mr Bush was understandably keen to play the good host when he met Mr Wen - even if it meant having to put down an old friend to do so. According to the Chinese premier, the two leaders agreed during their meeting that America and China had the most important state-to-state relationship in the world. Right now, Taiwan is not worth upsetting that relationship.

Even so, Mr Bush's statement is more a change of style than substance. Washington still opposes any change in the status quo that is opposed by either side. To keep the balance, it is not only giving Beijing verbal ammunition, it is also continuing to supply fighter planes and missiles to Taipei.

But Mr Chen has been warned that there are limits to Washington's support for Taiwan's democracy. Mr Bush, it seems, is far more interested in keeping the smile on Mr Wen's face.